This invention relates to Raman optical amplifiers, and more specifically to a multifrequency Raman amplifier pump source with a variable output spectral gain.
In a conventional long haul optical network it is often necessary to traverse distances of over one hundred kilometers between optical nodes. In order to propagate optical signals over such long distances it is necessary to provide energy to the optical signals. Typically this amplification is done with an erbium doped fibre amplifier (EDFA). A typical EDFA includes a pump source and length of erbium doped fibre. The pump source supplies energy to the erbium doped fibre. When an optical signal propagates within a region of erbium doped fibre during pumping, the erbium doped fibre will transfer some energy to the optical signal. Thus, in an EDFA, the amplification takes place within the length of erbium doped fibre.
One alternative to the EDFA that has gained popularity is the use of a Raman effect amplifier. Unlike an EDFA, a Raman effect amplifier uses the optical fibre to provide energy to the optical signal. These devices are generally more costly than EDFAs however Raman amplification is often used in undersea applications because it allows longer transmission distances that conventional EDFAs. In operation, the Raman pump provides a pump signal that propagates in a direction opposite the propagation of the optical signal that it is amplifying. When using an EDFA to amplify an optical signal, the gain provided to the optical signal is achieved in a short lengthxe2x80x94usually a few meters xe2x80x94of erbium doped fiber. Since Raman amplifiers use conventional optical fibre as a gain medium and the amplification is typically achieved over a distance of many kilometers. In some applications it is known to provide an EDFA and a Raman amplifier together. In this case, the Raman amplifier amplifies the optical signal prior to reaching the EDFA. The EDFA then boosts the optical signal again. This arrangement takes advantage of amplification from the comparatively inexpensive EDFA while still retaining some of the benefits of the additional distance provided by Raman amplification. In Raman effect amplification, light traveling within a medium is amplified by the presence of lower wavelength pump light traveling within the same medium. Typically maximum gain in silica fibers occurs at a frequency 13 THz lower than a relatively narrow Raman pump frequency band. The gain medium is either the transmission fiber itself, or a separate fiber optimized for Raman amplification.
Each Raman pump laser has a relatively narrow gain spectrum associated with it in the C band. Therefore a plurality of Raman lasers, spaced at fixed frequencies, are required to obtain continuous gain in the C band, as is obtained by using an EDFA. Additionally, a Raman amplifier will also allow the C band to be widened by supporting optical wavelength channels proximate the C band but having shorter wavelengths. Conventional methods of broadening the amplification in the C band comprise using a plurality of pump lasers at a spaced frequency coupled to a multiplexer such that a continuous gain spectrum is obtained within the gain fiber. Clearly, Raman effect amplifiers are very costly due to the number of required pump lasers and the components used to support the various lasers. In comparison a conventional EDFA uses one or a few pump lasers, depending on the application.
For conventional pump sources, laser diodes are coupled to optical fibers and require special lenses. The optical mode leaving a laser diode is elliptical in shape. In order to minimise the insertion loss between the laser diode chip and the optical fiber the elliptical mode is transformed to a circular mode prior to being provided to the fiber. This requires costly lenses for the mode conversion. Distributed feedback (DFB) lasers are useful for pump sources because of the frequency stabilizing grating used for feedback in to the cavity, however these lasers are expensive because a grating is either assembled within the laser cavity or attached externally to the laser after pigtailing.
Prior art U.S. Pat. No. 6,055,250 entitled xe2x80x9cMultifrequency Laser Having Reduced Wave Mixingxe2x80x9d details the use of a plurality of multifrequency sources coupled to a shared waveguide grating in order to achieve a multi frequency output such that any mixing signals on the output do not overlap in frequency. It is a form of a commonly referred to MAGICxe2x80x94multi-stripe array grating in a cavityxe2x80x94laser. This patent does not teach the use of the integrated multifrequency lasers for EDFA or Raman pumping amplifications.
It would be advantageous to manufacture an inexpensive laser source coupled directly to a waveguide device such that frequency stabilization feedback comes from a single shared grating instead of a plurality of gratings within each laser.
It would be advantageous to provide a single laser cavity having multiple laser diode sources within the cavity such that each of the laser diodes obtains frequency feedback from a same grating, whereby the intensity of each source is variable in such a manner as to shape the gain spectrum.
It would be beneficial to produce an inexpensive Raman effect amplifier.
In accordance with the present invention a multifrequency Raman pump laser cavity is disclosed comprising of an angularly dispersive element optically coupled to a shared waveguide terminated in a partially reflecting facet; a plurality of laser diode sources for radiating at frequencies for Raman amplification each for providing laser light at a different frequency and spatially oriented in relation to the angular dispersive element such that light emitted from each source is reflected from the angularly dispersive element toward a same output port; at least two partially reflecting coating for forming a laser cavity therebetween wherein the angularly dispersive element is within the laser cavity or defines a boundary thereto.
Similarly, the invention provides a multifrequency EDFA pump laser cavity comprising, a plurality of light sources having a gain medium associated therewith for radiating at frequencies for EDFA amplification each for providing laser light at a different frequency and spatially oriented in relation to an angular dispersive element such that light emitted from each source is reflected from the angular dispersive element toward a same output port; and, at least two partially reflecting coatings for forming a laser cavity therebetween wherein the angular dispersive element is one of within the laser cavity and defining a boundary thereto wherein at least two of the plurality of laser diode sources, the angular dispersive element and the at least two partially reflecting coatings are disposed on a same substrate.
The invention also describes a multifrequency pump laser cavity comprising, a plurality of light sources having a gain medium associated therewith for radiating at a first set of frequencies for EDFA amplification and a second set of frequencies for Raman amplification each for providing laser light at a different frequency and spatially oriented in relation to an angular dispersive element such that light emitted from each source is reflected from the angular dispersive element toward a same output port; and, at least three partially reflecting coatings for forming a laser cavity therebetween wherein the angular dispersive element is one of within the laser cavity and defining a boundary thereto wherein at least two of the plurality of laser diode sources, the angular dispersive element and the at least three partially reflecting coatings are disposed on a same substrate.